1. Field of the Invention
The present invention relates generally to infrared image sensors for detecting infrared rays radiated by objects to take distributions of temperatures and emissivity of the objects as image and, more particularly, to a Schottky type infrared image sensor having improved efficiency in removing undesired light.
2. Description of the Background Art
Image pick-up by using an infrared image sensor is widely applied to various industrial measurements and monitoring and in a field of research and development. A conventional infrared image pick-up apparatus is a mechanical scanning type using a single element or one dimensional small-sized arrays. In recent years there has been rapid progress in the semiconductor technology, enabling two dimensional arrays of practical level to be obtained.
Conventional examples of Schottky type infrared image sensors having arrangements including so-called cold shield are shown in FIG. 4 on page 48 of "Proceedings of the SPIE (1983), Vol. 443, Infrared Detectors" and in FIG. 10 on page 165 of "SPIE Vol. 302 Infrared Technology for Target Detection and Classification (1981)".
FIG. 1 is a sectional view of an infrared image pickup apparatus mounting an infrared image sensor 1 of approximately the same type as that of the above-described conventional infrared image sensor. This infrared image sensor 1 has integrated infrared detecting elements disposed one- or two-dimensionally and as shown in the drawing, the sensor is disposed in an internal portion 2a of a vacuumed dewar 2. A cold shield 3 is disposed in the internal portion 2a of the dewar 2 to cover the peripheral portion above the infrared image sensor 1. This cold shield 3 is provided to ensure a fixed field of view with respect to incident infrared rays from a window 4 at the upper portion of the dewar 2 of the infrared image sensor and to prevent undesired incident light from the outside of the dewar 2 by cutting off thermal radiation from a body tube in an optical system or the like. Attached to lower ends of left and right leg portions 5a and 5b of the dewar 2 are lead-through electrodes 6a and 6b for externally extracting a clock power supply or signal output for driving the infrared image sensor 1.
A cavity portion 7 between the left and right leg portions 5a and 5b of the dewar 2 is filled with liquid nitrogen or the like for cooling the infrared image sensor 1. The infrared image sensor 1 is adhered to the internal surface of the dewar 2 and cooled by heat conduction through the dewar 2.
As indicated by the arrow A of FIG. 1, the infrared rays enter the infrared image sensor 1 through the window 4. However, the incident infrared rays on the image sensor 1 are not limited thereto but they are radiated from every portion constituting the image pick-up apparatus, among which a higher temperature portion radiates more. Such undesired incident light from other fields of view than the regular field on the infrared image sensor 1 increases noise and reduces a dynamic range. Provided for cutting off the undesired incident light is the cold shield 3. The cold shield 3 is set to be maintained at a temperature as low as that of the infrared image sensor 1 and to have a high internal emissivity and a lower external emissivity. The aperture of the cold shield 3 is configured as shown in FIG. 2. More specifically, assuming that B.sub.1 and B.sub.2 shown in FIG. 2 denote left and right end positions of an aperture stop for stopping down the incident infrared rays on the infrared image sensor 1, the solid angles .theta.a and .theta.c for the detecting elements 8 nearest to edges are smallest among .theta.a,.theta.b and .theta.c at which the detecting elements 8 provided on the lower main surface of the infrared image sensor 1 looking up at B.sub.1 and B.sub.2. Therefore, the aperture configuration of the cold shield 3 is settled such that the solid angles .theta.a and .theta.c become larger than the solid angle .theta.y of one detecting element required in optical designing of the image pick-up apparatus.
While three detecting elements 8 are provided for the sake of explanation in FIG. 2, numbers of miniaturized detecting elements are actually provided two-dimensionally.
As conventional art to resolve the above-described problem of the conventional example, proposed is Japanese Patent Laying-Open No. 63-43366 is the arrangement of the back-illuminated infrared ray sensor provided with infrared ray absorbing layers disposed selectively, wherein crystal of mercury.cadmium.tellurium of a first conductivity type is formed on a surface of a cadmium.tellurium substrate, on the surface of which formed at a predetermined interval are a plurality of regions of a conductivity type opposite to the first conductivity type and infrared ray absorbing layers are selectively provided between the cadmium.tellurium substrate and the crystal of mercury.cadmium.tellurium. For the infrared ray absorbing layer, a mercury.cadmium.tellurium layer of a thickness of about 1 .mu.m is used.
However, in the above-described arrangement of the conventional image pick-up apparatus, as the detecting elements increase in number, a chip size of the infrared image sensor 1 increases. Therefore, the aperture diameter of the cold shield 3 needs to be made larger in order to ensure enough solid angle in each detecting element in the peripheral portion thereof. As a result, the shield effect of the cold shield 3 is reduced to cause an increase in noise sensed by the sensing elements and saturation. More specifically, as shown in FIG. 3A, in the optical system of the above-described conventional arrangement, since for example, the solid angle of the incident rays onto the detecting element 8 at the center of the infrared image sensor 1 through a lens 10 of an apparatus stop 9 is smaller than the solid angle at which the aperture of the cold shield 3 is looked up at from the detecting element, the incident infrared rays in the hatched part of the drawing become interference light 11 for the detecting element. As the foregoing, the optical system is referred to as a non-matching optical system, the system wherein the regions at which the incident infrared rays pass through the aperture surface of the cold shield 3 vary depending on the positions of the detecting elements of the infrared image sensor 1.
A so-called aperture-matching optical system can be applied to resolve the problem of the above-described conventional non-matching optical system. FIGS. 3B to 3D show examples of arrangements of the aperture-matching optical system. The aperture portion of the cold shield 3 is an aperture stop of the optical system in each arrangement. Since the aperture stop determines the extent of the signal rays passing through the optical system, the body tube can not be seen from each detecting element through the aperture of the cold shield 3. Namely, the undesired infrared rays irradiated from the body tube can be cut off by the cold shield 3. Out of these aperture-matching optical systems, the arrangement shown in FIG. 3B is an example wherein the aperture stop 9 of the conventional optical system is replaced by the cold shield 3, with the lens 10 provided at the aperture portion of the cold shield 3. In the optical system of this type, the lens 10 is included in the cold shield 3 to increase heat load of the cooling means.
The optical system shown in FIG. 3C is the example wherein the aperture portion of the cold shield 3 is arranged at the light emission side of the lens 10. With this arrangement, while optical designing is restricted, there is no such problem of cooling as in the arrangement of FIG. 3B. However, in the optical system of this type, a region of the lens through which the incident rays pass varies so largely depending on the solid angles that effective diameter of the lens becomes large.
The optical system shown in FIG. 3D is the example of the arrangement comprising an objective lens 10a and a relay lens 10b. The relay lens 10b forms on the infrared image sensor 1 the target image formed by the objective lens and forms the image (exit pupil) of the aperture stop 9 of the objective lens 10a on the aperture surface of the cold shield 3. The aperture of the cold shield 3 is set to be smaller than the exit pupil such that the aperture stop of the objective lens can not be seen from the detecting elements. In the optical system of this type, the diameter can be small but the total length is large.
With the infrared ray sensor disclosed in Japanese Patent Laying-Open No. 63-43366, it is difficult to form infrared image sensors of a monolithic arrangement on the same substrate, so that detecting elements and reading circuits are separately manufactured and put one upon another to have a hybrid structure. Therefore, the increased number of pixels makes the manufacture thereof difficult.